Military activities greatly increase the risk for osteoarthritis (OA). Knee OA, a leading cause of disability, is highly prevalent in Veterans. Current treatment options are limited primarily to palliation until the joint is destroyed and prosthetic knee replacement is warranted. Disease-modifying and preventive treatments are lacking in large part due to a lack of early warning tests that can alert patients and caregivers of the risk of OA onset prior to development of irreversible changes. Articular cartilage does not have nerves, and thus OA is largely asymptomatic at early stages when cartilage wear begins. By the time symptoms occur, the disease has typically reached an advanced state where irreversible cartilage loss and bone changes are present. Successful intervention requires early detection when pathological changes remain reversible. Serum-derived biomarkers offer an attractive method to provide early warning of joint distress, and to provide more rapid assessment of the efficacy of disease-modifying therapies than established methods such as radiographs. However, development of a blood test reflecting OA risk has been elusive. Some of the barriers include large inter-subject variability in the nominal resting levels of serum biomarkers as well as the limited ability of systemic markers to reflect local joint conditions. These factors reduce the sensitivity of measuring serum biomarkers in isolation. This study will apply the use of a mechanical stimulus (a 30 minute walk) in the evaluation of serum biomarkers (a stimulus-based approach), to generate preliminary data for the development of an OA stress test to detect early disease and to evaluate the efficacy of new disease-modifying strategies for OA. Similar to what is used in other fields of medicine such as cardiology, where greater accuracy and sensitivity in detecting underlying pathology can be achieved in response to specific provocative stimuli than is possible at a resting state, the use o a walking task to challenge knees with early OA may improve the signal-to-noise ratio for serum markers of cartilage and bone turnover. Aim 1 of the project will recruit Veterans with symptomatic early knee OA to investigate the use of this novel stimulus-response model to measure the change in serum biomarkers after a walking challenge for identification of early disease states compared to that of structural changes assessed using both morphological MRI and advanced image analysis techniques to measure cartilage matrix structure using quantitative MRI. Aim 2 of the project will use a prospective study design to investigate the ability of the OA stress test to provide an early indication of patient response to a simple and inexpensive load-modifying therapy (variable-stiffness shoe) for medial knee OA. Success of these Aims will provide evidence that serum biomarkers analyzed in relationship to a mechanical stimulus may be able to detect disease at an early enough time point for disease-modifying therapies. In addition, the success of these Aims will lay the groundwork for further clinical studies to determine whether this stimulus-response strategy may shorten the time needed for objective measures of outcomes following an interventional strategy from several years using radiographs to potentially a few months measuring biologic responses to treatment. It is anticipated that the results of this proposal will provide key preliminary data for the use o a stimulus-based approach to OA biomarkers (an OA stress test), which may enhance the ability to use serum biomarkers in knee OA diagnosis and clinical trial design. The use of such an OA stress test has the potential to provide early warning of the initiation of cartilage matrix breakdown and prompt earlier treatment to slow down the processes leading to OA. The results of this study will contribute rigorous preliminary clinical data important to the treatment and rehabilitation of military service members and Veterans, a population bearing a disproportionate burden from premature disabling OA.